1. Field of the Invention
This invention relates to sign post constructions, and particularly to a highway sign post construction that has a breakaway joint for safety purposes. The invention represents an improvement over applicant's earlier U.S. Pat. No. 5,125,194.
2. Prior Art
Federal, state and local governments require that sign posts and other structures associated with road construction be of a type designed to insure the safety of motorists in the event of a collision with the sign posts or other structures. Specifically, the Federal Highway Administration (FHWA) sets standards and oversees the design and construction of traffic sign posts on federal highways. Sign supports must also conform to standards set by the National Cooperative Highway Research Program (NCHRP) and American Association of State Highway Transportation Officials (AASHTO).
As determined by tests and usage experience, breakaway sign posts have proven to be highly effective in reducing vehicle damage and occupant injury resulting from collision with highway marker signs. A variety of breakaway sign post constructions intended to meet safety requirements for highway installations are therefore used in the prior art for installation of highway traffic signs.
Conventional breakaway connections are typically provided between a relatively short section of post (base post or ground post) that is driven into the ground, and a longer section of post (support post) that extends upwardly above the ground from the base post and supports the sign. When a motor vehicle collides with the sign post or posts, the section of the sign post above the ground is typically sheared off or hinged over to allow the motor vehicle to continue on its path with minimum damage to the vehicle and without injury to its occupants. Federal and state law typically require that the ground post extend no more than four inches above the ground following a collision to avoid damage to fuel lines or penetration of the vehicle passenger compartment as the vehicle passes over the broken-off posts.
One particular type of sign post that has been used extensively in the prior art comprises a steel U-channel having a flat, longitudinal center web with angularly outwardly divergent side walls terminating in lateral flanges. Spaced apart holes are provided through the center web along the length of the sign post to permit mounting of a sign to the post at a selected height with one or more suitable fasteners, such as bolts or screws, and to accept fasteners for securing sections of the post together to provide the breakaway joint.
It is important that a predictable and reliable breakaway splice or connection be provided between the post sections to insure that the sign post will break without imparting excessive force to the vehicle or causing injury to its occupants, and to insure that the stub height of the ground post does not cause damage to the vehicle as the vehicle passes over the ground post following a collision. It is also important that such sign posts be capable of withstanding ambient wind loads normally encountered by highway sign installations. Experience has shown that signs supported by a single support post, especially, tend to flutter when subjected to wind loads, and this fluttering action imposes torsion on the post and fasteners holding the post sections together. This torsional loading of conventional sign posts and fastener systems leads to frequent failure of the fasteners and/or posts. Evidence indicates that in the majority of failures due to wind load, the top bolt fails first, followed by failure of the U-channel post in the vicinity of the splice.
Prior to the invention described in applicant's earlier U.S. Pat. No. 5,125,194, U-channel post sections were secured together either without any spacers in the spliced joint or with improper spacers. This resulted in the load being carried by the fasteners and by contact between the sides of the U-channel. Because of the configuration of such U-channel post constructions, the webs remain spaced apart, and tightening of fasteners extended through the splice resulted in deformation of the post sections. The result was unpredictable failure of the sign posts in the event of a collision, and frequent failure due to torsional stress transferred to the fasteners under varying wind loads on the sign carried by the post. Applicant's earlier invention provided spacers between the U-channel sections, preventing deformation of the posts when the fasteners are tightened, and insuring predictable behavior of the spliced connection.
Further, prior to applicant's earlier invention it was very difficult for one person to assemble a sign post from upper and lower sections, since that person was required to extend multiple fasteners through the lower post section, hold the upper post section in proper orientation while positioning it over the fasteners, without dislodging the fasteners, and then secure the fasteners by placing and tightening washers and nuts on them, for example, all while holding the upper post section in proper upright position. Applicant's earlier invention of threaded spacers for engagement on the fasteners solved this problem.
However, it was found that many contractors installing highway sign posts frequently utilized improper spacing between the fasteners, even when using applicant's threaded spacers, leading to unpredictable behavior of the splice and premature failure due to ambient wind loads on the installation.
Moreover, the use of multiple, spaced apart spacers in the prior art installations, including that disclosed in applicant's U.S. Pat. No. 5,125,194, resulted in an area between the two fasteners that was effectively hollow, with two substantially solid bearing areas located at the fasteners. This produced two spaced apart relatively small bearing areas.
Accordingly, there is need for a highway sign post breakaway connection that is easy to install and predictable in operation, with means to insure that the fasteners used to couple the sign post sections together are properly spaced, and with means providing uniform and maximum bearing area between the nested U-channel post sections at the spliced connection to ensure proper strength in the joint and concentration of force in the fasteners so that predictable failure occurs in the fasteners rather than in the post sections.